The invention relates to absorbance modulation, a method of achieving sub-wavelength optical imaging resolution by forming radiation patterns on a photochromic layer at two wavelengths, an “imaging” wavelength and a “masking” wavelength, wherein the masking wavelength induces optical absorbance (opacity) in the layer so that the imaging wavelength only transmits through narrow optical windows defined by intensity nulls in the masking-wavelength radiation.
The method is primarily applicable to lithography (absorbance modulation optical lithography), in which the transmitted imaging-wavelength radiation exposes a “printing surface” (i.e., a photosensitive recording medium) proximate the photochromic layer. (The “imaging” wavelength can alternatively be termed the “exposure” wavelength in the context of lithography.) This type of system is described by R. Menon and H. I. Smith in “Absorbance-modulation optical lithography,” Journal of the Optical Society of America A, Vol. 23, Issue 9, pp. 2290-2294 (2006).
The method is also applicable to microscopy (absorbance modulation optical microscopy), in which the imaging-wavelength radiation interacts with an inspection surface proximate the photochromic layer and is collected by optical detection means to acquire image information about the surface. A system of this type is described by H. Tsai, E. E. Moon, and R. Menon, in “Far-Field Optical Imaging at the Nanoscale via Absorbance Modulation,” Novel Techniques in Microscopy, OSA Technical Digest (CD) (Optical Society of America, 2009), paper NMA2.
Prior-art absorbance modulation systems typically employ a binary-optic, zone-plate microlens array to concentrate the radiation onto points of a focal-point array proximate the photochromic layer, as described by R. Menon, P. Rogge, and H. Tsai in “Design of diffractive lenses that generate optical nulls without phase singularities,” Journal of the Optical Society of America A, Vol. 26, Issue 2, pp. 297-304 (2009). At each focal point a focused-radiation spot at the imaging wavelength is superimposed with an annular-radiation spot at the masking wavelength. Each annular-radiation spot has an optical null at its center, which induces a small, sub-wavelength transmittance window in the photochromic layer through which the imaging-wavelength radiation passes. (“Sub-wavelength” means smaller than the imaging wavelength.) A raster-scanning method is employed to synthesize a full-field lithographic or microscopic image from the focal-point array. In the context of lithography, the focused-radiation spots are modulated by means of a spatial light modulator in a maskless lithography process such as the Zone-Plate Array Lithography (ZPAL) system manufactured by LumArray, Inc. of Somerville, Mass.
Alternative types of microlenses other than binary-optic zone plates can be used for maskless lithography. These include blazed, phase-Fresnel lenses (U.S. Pat. No. 6,960,773) and refractive microlenses (U.S. Pat. No. 6,133,986). However, the use of such alternatives for absorbance modulation does not appear to be practiced in the prior art. Also, more conventional projection optics, which do not employ an image-plane microlens array, have not been adapted for use with absorbance modulation.